In process measurements and automation technology, for the highly accurate measurement of physical parameters, such as e.g. mass flow, density and/or viscosity, of a medium flowing in a pipeline, for example a gas and/or a liquid, often such inline measuring devices are used, which, by means of a measurement pickup, or transducer, of vibration-type, through which the medium is flowing, and a measurement and operating circuit connected thereto, effect reaction forces in the medium, forces such as e.g. Coriolis forces corresponding with mass flow, inertial forces corresponding with density, or frictional forces corresponding with viscosity, etc., and produce, derived from these, a measurement signal representing, for the medium, mass flow, viscosity and/or density, as the case may be. Such inline measuring devices with a measurement pickup of vibration-type, as well as their manner of operation, are known per se to those skilled in the art and are described comprehensively and in detail e.g. in WO-A 03/095950, WO-A 03/095949, WO-A 02/37063, WO-A 01/33174, WO-A 00/57141, WO-A 99/39164, WO-A 98/07009, WO-A 95/16897, WO-A 88/03261, US 2003/0208325, U.S. Pat. Nos. 6,880,410, 6,691,583, 6,651,513, 6,513,393, 6,505,519, 6,006,609, 5,869,770, 5,861,561, 5,7 96,011, 5,616,868, 5,602,346, 5,602,345, 5,531,126, 5,359,881, 5,301,557, 5,253,533, 5,218,873, 5,069,074, 4,876,898, 4,733,569, 4,660,421, 4,524,610, 4,491,025, 4,187,721, EP-A 1 291 639, EP-A 1 281 938, EP-A 1 001 254 or EP-A 553 939.
For the conveying of the flowing medium, the measurement pickups include, in each case, at least one measuring tube with a bent and/or straight tube segment, which is caused, during operation, to vibrate, driven by an electromechanical exciter mechanism, for producing the above-mentioned, reaction forces. The measuring tube is held in a support frame most often embodied as a closed, pickup housing. For registering vibrations, especially inlet-side and outlet-side vibrations, of the tube segment, the measurement pickups further include, in each case, a sensor arrangement reacting to movements of the tube segment.
In the case of Coriolis mass flow measuring devices, measurement of the mass flow of a medium flowing in a pipeline rests, as is known, on the allowing of the medium to flow through the measuring tube inserted into the pipeline and oscillating during operation, at least in part, laterally to a measuring tube axis, whereby Coriolis forces are induced in the medium. These forces, in turn, effect, that inlet end and outlet end regions of the measuring tube oscillate shifted in phase relative to one another. The size of these phase shifts serves, in such case, as a measure for the mass flow rate. The oscillations of the measuring tube are, therefore, registered by means of two oscillation sensors of the aforementioned sensor arrangement spaced from one another along the measuring tube and transformed into oscillation measurement signals, from whose phase shift with respect to one another the mass flow is derived. Already the initially referenced U.S. Pat. No. 4,187,721 mentions, additionally, that also the instantaneous density of the flowing medium is measurable by means of such inline measuring devices, and, indeed, on the basis of a frequency of at least one of the oscillation measurement signals delivered by the sensor arrangement. Moreover, most often, also a temperature of the medium is directly measured in suitable manner, for example by means of a temperature sensor arranged on the measuring tube. Additionally, straight measuring tubes can, when excited to torsional oscillations about a torsional oscillation axis essentially extending parallel to, or coinciding with, the pertinent measuring tube longitudinal axis, effect that radial shear forces are produced in the medium conveyed therethrough, whereby, in turn, significant oscillatory energy is withdrawn to the torsional oscillations and dissipated in the medium. As a result thereof, a considerable damping of the torsional oscillations of the oscillating measuring tube occurs, so that, to maintain the torsional oscillations, additional electrical exciting power must be supplied to the measuring tube. Derived from a correspondingly required electrical exciting power for the maintaining of the torsional oscillations of the measuring tube, it is possible, in manner known to those skilled in the art, so also to determine, at least approximately, a viscosity of the medium; compare, in this connection, especially also U.S. Pat. Nos. 4,524,610, 5,253,533, 6,006,609 or 6,651,513. It is possible, therefore, to assume, without more, in the following that, even when not expressly described, modern inline measuring devices with measurement pickups of vibration-type, especially Coriolis mass flow measuring devices, can be used, in any case, also to measure density, viscosity and/or temperature of the medium, especially since these variables are often drawn upon, in the case of mass flow measurement, in any event, for the compensating of measurement errors arising due to fluctuating medium density and/or viscosity; compare, in this connection, especially the already mentioned U.S. Pat. Nos. 6,513,393, 6,006,609, 5,602,346, WO-A 02/37063, WO-A 99/39164 or also WO-A 00/36379.
Besides such measurement pickups, or transducers, of vibration type, frequently also inline measuring devices with magneto-inductive pickups, or transducers, or measurement pickups, or transducers, evaluating the travel time of ultrasonic waves emitted in the stream direction, especially also those working on the basis of the Doppler principle, are used in process measurements and automation technology for inline measurements. Since the principles of construction and manner of functioning of such magnetic-inductive measurement pickups are described adequately e.g. in EP-A 1 039 269, U.S. Pat. No. 6,031,740, 5,540,103, 5,351,554, 4,563,904, etc., and the principles of construction and manner of functioning of such ultrasonic measurement pickups are described adequately e.g. in U.S. Pat. Nos. 6,397,683, 6,330,831, 6,293,156, 6,189,389, 5,531,124, 5,463,905, 5,131,279, 4,787,252 etc., and, moreover, are likewise sufficiently known to those skilled in the art, a detailed explanation of these principles of measurement can be omitted here.
In the use of such inline measuring devices comprising at least one measuring tube joined into the course of the pipeline conveying the medium, it has, however, been found, that, in the case of inhomogeneous media, especially two, or more, phase media, the measurement signals produced therewith can be subject, to a considerable degree, to non-reproducible fluctuations, even though the medium parameters significantly influencing the measurement signals, especially the mass flow rate, are held essentially constant; compare, in this connection, also the initially mentioned U.S. Pat. No. 6,910,366, U.S. Pat. No. 6,880,410, U.S. Pat. No. 6,505,519, U.S. Pat. No. 6,311,136 or U.S. Pat. No. 5,400,657. As a result, these measurement signals in the case of multiphase streams of medium are practically unusable for a highly accurate measurement of the physical flow parameter of interest. Such inhomogeneous media can be, for example, liquids, in which, as e.g. practically unavoidable in metering or bottling processes, a gas present in the pipeline, particularly air, is entrained, or from which a dissolved medium, e.g. carbon dioxide, outgasses and leads to foam formation. As further examples of such inhomogeneous media, also emulsions, wet, or saturated, steam, as well as fluids carrying solid particles can be mentioned.
Especially in the case of inline measuring devices comprising a measurement pickup of vibration-type, such as, for example, also discussed in JP-A 10-281846, EP-A 1 291 639, U.S. Pat. No. 6,880,410, 6,505,519, it has been found that the oscillation measurement signals derived from the oscillations of the measurement tube, especially also the mentioned phase shift, are, in the case of two, or more, phase media, and in spite of keeping the mass flow rate, as well as also viscosity and density in the separate phases of a medium, practically constant and/or appropriately taking such into consideration, subject to fluctuations to a significant degree and, therefore, in given instances, can be completely unusable for measuring the physical flow parameter of interest, unless remedial measures are undertaken. Mentionable as reasons for the measurement errors accompanying the measurement of inhomogeneous media by means of measurement pickups of vibration-type include, for example, the unilateral clinging or depositing of liquid-entrained, gas bubbles or solid particles on the measuring tube wall, and the so-called “bubble-effect”, in the case of which gas bubbles entrained in the liquid act as flow bodies for liquid volume elements accelerated transversely to the longitudinal axis of the measuring tube.
For lessening the measurement errors accompanying two, or more, phase media, U.S. Pat. No. 6,880,410, for example, proposes a flow, or medium, conditioning to precede the actual flow measurement. As another possibility for escaping the problems of such measurement pickups in connection with inhomogeneous media, for example, both in JP-A 10-281846, as well as also in U.S. Pat. No. 6,505,519, a correction of the flow measurement, especially of the mass flow measurement, determined from the oscillation measurement signals, is made. The correction is based especially on the evaluation of deficits between a highly accurately measured, actual density of the medium and an apparent density of the medium determined during operation using Coriolis mass flow measuring devices. Further methods for avoiding and/or correction of measurement errors associated with two, or more, phase media, beyond those mentioned above, are described in U.S. Pat. No. 2005/0081643, U.S. Pat. No. 2005/0022611, WO-A 2005/057137 or WO-A 2005/057131.
Alternatively or in supplementation thereof, additionally, also measurement systems, especially diversely operating measurement systems, formed by means of a plurality of inline measuring devices and disclosed e.g. in U.S. Pat. Nos. 5,400,657, 5,259,250, 2005/0016292 or WO-A 03/062759, WO-A 03/073047, WO-A 03/087735, or WO-A 04/046660, can be applied for measuring two, or more, phase media. However, a significant disadvantage of such, actually quite precisely measuring, measurement systems can reside in their increased complexity and the accompanying high installation costs, on the one hand, and the high servicing and maintenance costs, on the other hand. Moreover, such measurement systems possess, most often, a relatively high requirement for space.